O(g,nn) reactions at 23.7 and 32.0 MeV*

Photoneutron reactions on ¹⁸O offer an opportunity to examine the neutron-neutron interaction and may be used as a surrogate reaction for planning a ³H photodisintegration experiment at the TUNL.  The relatively large ¹⁸O(g,nn) reaction cross section allows measurements of differential cross-section data for neutron-neutron coincidences at various opening angles between neutrons similar to the ³H(g,nn) reaction without handling a radioactive gas target.  Cross sections for the sequential emission of neutrons in the ¹⁸O(g,nn) reaction are measured.   The sequential decay is the dominant portion of the cross section. Simultaneous neutron emission is highly suppressed due to Isospin.

Measurements were performed at the HIGS using a 23.7 and 32 MeV circularly polarized photon beam. Neutrons were detected using a neutron detector array comprised of 30 liquid scintillator detectors at reaction angles of 65, 90, and 135°. Neutron energies were determined using time-of-flight techniques. These are the first differential cross-section data for ¹⁸O(g,n) and ¹⁸O(g,nn). A description of experimental techniques, method for neutron cross talk subtraction and final results will be presented.